Progesterone receptor transiently introduced into mouse cells harboring a stable, episomal multicopy mouse mammary tumor virus (MMTV) long terminal repeat (LTR)-reporter fusion cannot activate stable, replicated copies of the MMTV promotor. Transiently introduced glucocorticoid receptor is quite efficient, indicating that the progesterone receptor (PR) is intrinsically defective in chromatin activation. Initial mapping of glucocorticoid receptor (GR)/PR chimeric molecules suggests a domain necessary for transactivation from chromatin can be identified. Using a new cross-linking technique, mapping of MMTV chromatin in vivo demonstrated that positioned cores on the LTR result from the frequency-biased occupancy of multiple frames, leading to a series of six nucleosome families across the LTR regulatory region. Cross-linking also showed that the B region chromatin transition does not involve nucleosome displacement. Nucleosome reconstitution experiments in vitro also demonstrated that B region cores can occupy multiple translational and rotational frames. The MMTV LTR was introduced into human mammary carcinoma cell lines containing endogenous PR. In these cells, B region chromatin was found to be constitutively hypersensitive, and the PR was shown to be bound to its sites in the absence of hormone. Thus, the PR can bind to DNA in vivo without hormone, and the receptor induced nucleoprotein transition can be uncoupled from basal complex interactions.